Coherent optical communication systems generally extract information from an incoming signal beam by superimposing a local oscillator beam on the received signal. In a typical coherent optical communication receiver, the received optical wave, which carries a temporal modulation to be detected, is combined with a locally generated optical wave, which is frequency shifted by a small amount from the received wave. The combined optical waves are directed to a photodetector to detect a beat signal that is proportional to the temporal modulation.
A problem arises, however, when the received wave front is not well matched with the wave front from the local oscillator. Phase distortions, which are especially harmful, result in low signal-to-noise ratios from the photodetector. As a practical matter, all optical wave fronts become distorted when propagating through a real atmosphere. As a result, coherent communication through the atmosphere has required the use of spatial filter systems that reduce the strength of the already weak received signals. The signal fading that results from these systems has made coherent communications restrictive and difficult when passing through distorting media. Thus, there is a need for optical systems that detect heterodyne beat frequencies to improve coherent optical communications.